8.4 Septic Tanks, Media Filters, Aerobic Treatment Units & Grease Interceptors
Key Takeaways
- G.S. 90A-38(b) exempts a conventional septic tank system — a settling tank and subsurface disposal field with no pump or other appurtenances — from the certified operator requirement, while systems with pumps, media filters, or ATUs are classified and require an operator.
- Septic tank/media filter systems are Grade I biological systems under 15A NCAC 08G .0302(a), and domestic systems treating 1,500 gallons per day or less are visited twice per year at six-month intervals — except ATUs of that size, which are visited weekly.
- A septic tank works by separating floatables into a scum layer and settleables into a sludge layer, leaving a clear zone in between; the tank must be pumped before solids carry over into the drainfield or media filter.
- Intermittent sand filters and recirculating media filters provide attached-growth treatment of septic tank effluent and are dosed rather than continuously loaded, which is what maintains aerobic conditions in the media.
- Grease interceptors are sized for retention and are only effective if pumped on a schedule; grease that passes into the collection system saponifies into deposits that are the leading cause of sanitary sewer overflows.
8.4 Septic Tanks, Media Filters, Aerobic Treatment Units & Grease Interceptors
The Biological Grade 1 needs-to-know gives three hours to septic tank and sand filter systems and half an hour each to grease traps and collection systems — a reflection of what Grade I operators actually manage in North Carolina.
1. When is a certified operator required?
G.S. 90A-38(b) is explicit: "No certificate shall be required under this Article to operate a conventional septic tank system," defined as "a subsurface sanitary sewage system consisting of a settling tank and a subsurface disposal field without a pump or other appurtenances."
Add a pump tank, a media filter, an ATU, a drip field, or a low-pressure pipe system and the exemption disappears: the system becomes classified and needs a designated ORC. 15A NCAC 08G .0307 classifies subsurface systems, and .0302(a)(1) makes septic tank/media filter systems Grade I biological.
Visitation: under 08G .0204(2)(e) a domestic system with a treatment capacity of 1,500 gpd or less is visited twice per year with a six-month interval; under (2)(f) a domestic ATU of that same size is visited weekly; and under (2)(g) systems permitted under Commission for Public Health rules follow 15A NCAC 18A .1961.
2. The septic tank
Inlet tee ----> | SCUM (grease, floatables) |
(dissipates |------------------------------------|
velocity) | CLEAR ZONE --> outlet tee/filter |----> to filter or field
|------------------------------------|
| SLUDGE (settled solids) |
- Function: separation and anaerobic digestion — not purification. BOD reduction is modest; solids removal is the point.
- Inlet and outlet tees (or baffles): the inlet tee dissipates velocity so it does not disturb the sludge; the outlet tee draws from the clear zone so scum cannot pass. A missing outlet baffle sends grease straight to the drainfield.
- Effluent filter: a screen on the outlet that catches carryover solids. It needs periodic cleaning back into the tank — a clogged filter causes a backup that customers blame on the field.
- Pumping trigger: pump when the sludge plus scum occupies roughly one-third of the tank's liquid depth, or when scum comes within a few inches of the bottom of the outlet device. Measure with a sludge judge and a scum stick rather than guessing.
- Compartments: two-compartment tanks improve solids retention; the second compartment protects against carryover during surges.
[!WARNING] Septic tanks are lethal confined spaces. Hydrogen sulfide and methane accumulate, and oxygen is displaced. Never enter a septic tank or pump tank — pumping and inspection are done from outside the tank. Multiple fatalities every year involve a rescuer entering after someone collapsed.
3. Media filters
| Type | Dosing | Notes |
|---|---|---|
| Intermittent sand filter (ISF) | Dosed several times daily onto the surface, then allowed to drain | Single pass; excellent BOD/TSS; needs surface raking and occasional media replacement |
| Recirculating sand/gravel filter (RSF) | Effluent recirculated 3:1 to 5:1 through a recirculation tank | Better nitrification and some denitrification in the recirculation tank; more tolerant of loading |
| Textile / foam / peat media | Dosed | Proprietary packaged units; lighter and smaller footprint |
Why dosing matters. A media filter is an attached growth process that depends on air moving into the media between doses. Continuous trickle loading floods the pores, drives the bed anaerobic, and causes ponding and odor. Dose volume and frequency — set by timer or demand float — are the operator's primary control.
Troubleshooting: ponding on the surface means clogging from solids carryover or biomass; uneven distribution means plugged orifices or a laterals leveling problem; odor means anaerobic conditions from overloading or inadequate venting.
4. Aerobic treatment units
An ATU is a small package plant — typically extended aeration — serving a home, cluster, school, or business:
- Components: trash/pretreatment compartment, aeration chamber with a blower or aspirator, clarifier with sludge return, and often a chlorine tablet feeder and contact chamber.
- Weekly visit checklist: blower operation and filter, aerator and diffuser condition, mixed liquor color and settleability (a 30-minute settleability in a bucket or cylinder is the field test), clarifier for solids carryover and rising sludge, sludge return function, tablet feeder charged, effluent clarity and residual, alarms functional.
- Sludge management: ATUs accumulate solids and must be pumped periodically — an ATU that has never been pumped will eventually wash its inventory over the weir.
- Why weekly. 08G .0204(2)(f) singles ATUs out because a blower failure turns an aerobic unit septic within days, and the only way to know is to look.
5. Pump tanks and dosing controls
Pump (dosing) tanks push effluent to a filter, drip field, or elevated disposal area. Operators check the float settings (off, on, high-water alarm), pump run times and cycle counts, the high-water alarm function, check valve and anti-siphon provisions, and the electrical panel for moisture and corrosion. A run-time meter and cycle counter are the best diagnostic pair available: rising run time at constant flow points to a failing pump or a restricted field, while rising cycle counts at constant use point to a leaking check valve or a float problem.
6. Grease interceptors and FOG control
Fats, oils, and grease discharged by food service establishments cool in the collection system, saponify with calcium into hard deposits, and combine with disposable wipes into the blockages that cause a large share of North Carolina's sanitary sewer overflows.
- Grease traps (indoor, small) serve individual fixtures; grease interceptors (outdoor, large) serve the whole kitchen and are sized for retention time.
- Effectiveness depends entirely on pumping. An interceptor that is not pumped on a schedule is simply a section of pipe. A common local ordinance standard is pumping when accumulated grease and solids reach 25 percent of the operating depth, on a documented schedule.
- A FOG program includes an ordinance, an inventory of food service establishments, permit or agreement terms, inspection, manifest recordkeeping for each pump-out, enforcement, and public education ("can the grease").
- Hot water and additives do not solve it. Emulsifiers and enzyme products move the grease downstream, where it cools and deposits in someone else's line; they do not remove it.
[!NOTE] Tie it to the SSO rules. When a grease blockage causes an overflow, the reporting obligations of G.S. 143-215.1C and the 24-hour visitation requirement of 08G .0204(2)(d) attach immediately — which is why FOG enforcement is prevention, not paperwork.
Under G.S. 90A-38(b), which system does NOT require a certified operator?
Why is an intermittent sand filter dosed in discrete volumes rather than loaded continuously?
A domestic aerobic treatment unit rated at 900 gallons per day serves a rural restaurant. How often must the ORC visit under 15A NCAC 08G .0204(2)?